tchéthache or tchéthasse, tchérache, etc. or tchiéthache, etc., s.f.
1. Thin, hard skin. From the old cow - it's nothing but hard skin...
2. By extension, and in a pejorative sense, the skin of humans. What has he got into his tchéthasse, then? - under his skin, in his head. We should say that the old folk often pronounced it tchiéthache.
Cf. F. cuirasse = figuratively, protective casing of certain animals.